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Subaru flat 4 port and direct injection, non-turbo | Cylinder misfire

When is a misfire not a misfire?

As ever, vehicle history is all important and so an interview with the customer confirmed starting and driveability to be normal (hot or cold) with no loss of performance, but did reveal his vehicle had been seen by another garage where No. 2 cylinder Spark plug, Ignition coil, and Direct injector had already been replaced! The owner added that the warning light would only illuminate when the engine was at normal operating temperature at low road speed.

Looking at the freeze frame data accompanying the fault code P0302, it did indeed support the customer’s version of events, recording a hot engine, low road speed, and a misfire count of 44 on number 2 cylinder!

Freeze frame data below

Vehicle Speed

1

MPH

Engine Speed

865

rpm

Calculate Load

23.1

%

MAF

2.87

gm/sec

Coolant Temp

88

C

Cylinder #2 Misfire Count

44

In an attempt to reproduce the fault code whilst looking at live data in our workshop, the vehicle actually demonstrated the misfire count seen above! However, the engine idle quality did not appear to suffer other than the characteristic idle of this engine type at 650rpm. How nice it is to have a symptom that wants to reveal itself on demand!

As this was too good an opportunity to miss a 4 gas analyser test was carried out whilst monitoring the live engine data and ensuring the misfire count was still evident. Our results were most strange as all emission readings were correct with no errors both at idle and 2500 rpm. HC 12PPM, CO 0.005%, CO2 14.44% and O2 0.4% at idle with the live data list recording 20 misfire counts on number 2 cylinder! One point to note here was the misfire count fell to zero on number 2 cylinder as soon as the engine speed was increased just 150rpm above idle (return to idle, the misfire count began to increase again).

Idle emissions with a misfire count of 20+

CO

% vol:

------

0.300

0.005

Ok

CO2

% vol:

------

------

14.44

------

COc

% vol:

------

------

0.01

------

HC

ppm vol:

------

------

12

------

O2

% vol:

------

------

0.42

------

LAMBDA

:

0.970

1.030

1.020

Ok

Live data at idle indicating misfire No2 Cylinder

Vehicle Speed

0

MPH

Engine Speed

652

rpm

Calculate Load

27.8

%

MAF

2.41

gm/sec

Coolant Temp

89

C

Cylinder #2 Misfire Count

20

Throttle Sensor Position

1.9

%

At this stage, the dilemma is a misfire code and count that continually returns for No.2 Cylinder only when warm, no misfire evident, misfire counts falls to zero above idle speed, and GOOD emissions! Looking deeper into the data list for any further evidence of a misfire nothing appeared to stand out, O2 signal values, fuel Trim, high fuel pressure,Â MAP, MAF, and pending codes all returned OK which leads me away from a possible fuelling or ignition issue.

Here then we appear to have a misfire only the PCM can detect yet has no effect on the engine running condition. Knowing fault code P0302 indicates cylinder misfire we need to know how the PCM calculates this information to conclude a misfire is present. This is calculated by the PCM monitoring variations in crankshaft rotational speed and linking the variations to the camshaft position sensors in order to identify the offending cylinder.

To throw a spanner in the works we could not be 100% sure which cylinder was in fact No.2! Our Technical data claimed No. 2 Cylinder was at the RH rear whilst our wiring diagram indicated LH front and so how could we prove which cylinder the PCM controlled as No.2? My mind was racing ahead here thinking maybe the previous garage had installed one of the correct parts but to the wrong cylinder!
Using the ‘cylinder cut’ feature of the Scan Tool and placing our Coil-on-Plug (COP) probe on top of both the direct injector and Igniter we could monitor the signals as the PCM switched these components ON-OFF (cylinder cut). Here we discovered conclusive evidence that No 2 Cylinder was in fact the LH front cylinder and the cylinder that had received the new components, DAMN!Â

The cylinder cut feature of the Scan tool also proved inconclusive as each “cylinder cut” returned an equal fall in engine speed. PicoDiagnostics was used to carry out a non-intrusive compression test. At last our first breakthrough to prove something was not right! There is a saying that goes K.I.S.S (Keep. It. Simple. Stupid) and how true is that? One might argue why was this test not carried out at the start, and I would have to agree, but we had no misfire symptom other than a code and data list count.

NO2 direct injector non-intrusive test

NO2 igniter non-intrusive test

Here we have confirmation of a potential compression issue on one of the 4 cylinders. Please note the results appear to show a near 50% reduction in compression of one cylinder. This is a relative compression test where the 4 cylinders are compared against each other. My first thoughts were a cylinder that had a 50% reduction in compression would surely demonstrate more obvious symptoms, but we have to remember the engine speed here is only 200RPM (cranking speed). One of the symptoms highlighted a misfire count at low engine speed only, anything above idle and the misfire count fell to zero!

So we now had ammunition to go deeper regarding compression and of course the next step had to be “what cylinder is suffering from the loss of compression?”.

Here we opted for a cranking current test using PicoScope with the “Coil on Plug” (COP) probe placed on what we know confirmed as No. 2 Cylinder igniter assembly (no surprises here, No2 cylinder has loss of compression). Again another non-intrusive and simple test to provide evidence that will allow us to proceed with confidence and form the basis for a portfolio to present to the customer as this was starting to look very expensive!

The above waveforms provide the conclusive evidence that not only is there a loss of compression whilst cranking but we have now discovered No.2 Cylinder is the offender.

Knowing we have a loss of compression we had to discover where before we dive into compression testing and dismantling the engine. Here two WPS500 pressure transducers were used, 1 installed into the exhaust and 1 into the inlet manifold with the COP probe placed over No.2 igniter in order to relate the waveforms to a firing order. With the engine idling this revealed an area of real concern and eliminated another!Â The pulsations measured via the exhaust confirmed no excessive peaks so for now at least could be overlooked.Â However the intake pulsations via the intake manifold revealed an area of concern for inspection when dismantling the engine (see below).

Every 4th intake manifold pulsation peak to peak proved to be low, which again linked to No.2 cylinder. By aligning the No. 2 cylinder firing event we are able to locate the offending intake pulsation as the suspect intake pulsation takes place before the ignition event on No.2 cylinder. A further test was carried out again using two WPS500 pressure transducers this time via the intake manifold and the dipstick tube. The dipstick tube pulsations were all even so confirming no issues with excessive crankcase pressure hence piston and ring efficiency appears fine.

To recap, it is worth mentioning here that all the above tests have been non-intrusive other than removing an intake manifold hose, yet we have been able to deduce we have a compression issue on No.2 cylinder that appears to be related to the intake stroke. Equally we have also been able to confirm areas that are not of concern, for example Fuelling, H.T, piston to cylinder sealing, and exhaust valve sealing efficiencies.

Now we have the confidence and more importantly the evidence to remove the LH Spark plugs which on this vehicle is no mean feat at over an hour labour, highlighting the value of non-intrusive testing.

The WPS500 pressure transducer was then installed into the LH rear (NSR) cylinder and the waveform acquired by cranking and running the engine, saved and kept as a reference to compare against the LH front No.2 cylinder (NSF) under the same conditions.

Here we can see the reduced peak compression pressure of the NSF cylinder (No.2 Dark blue) in comparison to the under laid reference waveform of the NSR cylinder (Light blue). No.2 Cylinder peak compression achieved 9.77 bar compared to the NSR at 12.75 bar, a difference of 2.98 bar as indicated by our relative compression test earlier. Using the phase markers of the PicoScope Beta software we were able to mark out the Zero (TDC) and 720 degree (TDC) points of crankshaft rotation and then divide the distance between the phase markers equally using the phase marker partitions, enabling us to plot the events of the 4 stroke cycle.

What became very apparent was the lack of “Ripple/Turbulence” during the intake stroke of No.2 cylinder in comparison to the reference waveform.

We always have to look for differences or anomalies between known good signals and here we can see good turbulence from both cylinders during the exhaust stroke indicating good airflow dynamics but a definite reduction in turbulence of the air flow during No.2 cylinder intake stroke. We can also identify the early and prolonged opening of No.2 cylinder inlet valve in comparison to the reference waveform and the reduction in peak vacuum, all linked to the condition revealed below.

The above engine varies the valve timing according to engine load conditions. During cranking and idling, valve overlap is eliminated to improve idle quality.

At this stage enough is enough as no further testing can be carried out with the engine installed. Sufficient evidence had been gathered and presented to the customer confirming we were now looking at a fault of a mechanical nature. Permission was then given to remove and dismantle the engine that finally revealed near zero valve clearance with No. 2 Cylinder Inlet valves at 0.03 mm and 0.00 mm. This confirmed the misfire that proved evident only when warm and why the misfire would be more pronounced at low engine speeds as the valve would have been seeping. As engine speed increased both the momentum and increase in pressure would disguise the misfire (no misfire count at 150rpm above idle but near 50% misfire during relative compression test at 200 rpm). Looking back at the testing carried out the inlet valve could only have been marginally miss-seated as no misfire could be detected using cylinder cut or emission methods, yet the PCM had identified an anomaly with crankshaft rotational speed as did the relative compression test. The engine has since been reassembled with an inlet valve clearance of 0.12 mm and all is well with no misfire count or code apparent.

Comments

good case. i have a question, there is no hydrulic lift for this engine?

Ben StocktonFebruary 02 2015

Nice write up Steve. Great example of diagnostic logic.

AnonymousNovember 06 2014

Great and accurate article about the potential of the WPS!

BogdanOctober 13 2014

Very good writeup !
So much to learn from this !
Please keep them coming !

ELMAOULIJuly 11 2014

Really nice demonstration of how analayse data,using PicoScope

Edward PascoMay 06 2014

Excellent article

stuart bannertonApril 01 2014

I was born in a garage I am old school and would have done a compression test from start and checked if the right plugs were fitted as I work on Renaults and rong plugs do strange things and even put lights on dashes but as you said its no easy task removing the plugs but I would like to know how long all those tests took as you said removing the plugs takes an hour thanks stuart

RimvydasFebruary 19 2014

Good job,and it seems to me that it took much more time than to write this case study. Good luck.

Stuart G.February 13 2014

What a great example of using Pico and the Pressure tranducers! Normally you would grab for the compression tester, but using the pico it really shows compression in a whole new way. The use of the dipstick tube for crankcase leak was also really clever. Well done!

LaurentJanuary 29 2014

Great troubleshooting; just one comment on the compression wave: the TDC is not at peak (maximal pressure), but just before. In fact, pressure is maxi when piston and rod have 90Â° angle, but this is not at TDC.

Derek WarwickJanuary 27 2014

What a wonderful article showing the correct working practice and meticulous attention paid to the system involved and how by the right approach and correct testing methods as well as the right equipment, the indisputable conclusion is achieved. Very well done!

AnonymousJanuary 25 2014

Some bill for checking valve clearances

Dario CaraballoJanuary 24 2014

Congratulations, the way this diagnosis was developed shows great skills in using tools put at our disposal by Pico technology… Congratulations again to the technical team and the Pico technology staff for the development of this software and accessories

Stuart DuffJanuary 22 2014

Great use of the pico in nailing this one down, these sort of jobs are becoming more common and having the pressure transducers and more importantly being able to understand the readings can save you hours of work. Good job.

Andres AriasJanuary 22 2014

Very good case study, once again the importance of an oscilloscope is noted but even more important the importance of knowing how to use it, very well done, congratulations !!!

c jonesJanuary 22 2014

surly a comprission test would shown this at the very begining???

KikeJanuary 22 2014

Good story and well documented, love these stories! Question remains what may have caused this sudden clearance change, in other words the cause of the problem?

old_robinJanuary 21 2014

Nice to see a thorough and logical analysis of the problem rather than dashing in to change components almost at random. Good job!

Jim GilmourJanuary 21 2014

Brilliant sequenced diagnosis.

Andrew FowkesJanuary 21 2014

That diagnosis and subsequent write up (above) are a thing of beauty!
Brilliant.

GEORGE MBANJEJanuary 21 2014

with this demostration of the pico,i cant wait to go to the next available course date,as i have bought the equipment.

allan murrayJanuary 21 2014

Thank you for sharing a very well explained and proffesional approach to a incrediable problem. Have struck the same on the small Suzuki jimmy vehicles where dirty air cleaner lead to wearing away valve seat clearance and made very hard starting when hot, but motor ran with no problem when going.

In reading this, I kept thinking you were going to end up with a classic Subaru Valve Guide problem like they used to have once in a while.

audiyadoJanuary 21 2014

We came accross this same fault about 5 years ago and again last year, Both with Volvo’s. We checked ignition and fueling side with scope found slightly high KV’s on suspect cylinder. Did balance check & comparative compression one cylinder down on both engines. Then I took a reading at the MAF whilst conected to the camshaft sensor and found large back flow on one cylinder each revoloution (Using cam signal isolated cylinder in firing order with trouble) Now knew we had a bad inlet valve. Sticky valves on both engines! Well done for getting through this as our customers would not have that it was an engine fault and not electrical until proved wrong!!!

Matt WilliamsJanuary 20 2014

Extremely well written article Steve and what a great way to demonstrate the capabilities of the product!

David PatersonJanuary 20 2014

A superb write-up with attention to some important, if basic detail. This article really shows how the oscilloscope - and its accessories - lends itself so well to confirming basic mechanical defects, and that it isn’t just a tool for electrical problems.
In the 80s we used a Sun oscilloscope the size of a wardrobe and rolling road to hunt down problems. I learned then not to overlook the basics and indeed this was a common problem, since there was always a tendency for many repairers â€“ including concessionaires - to look for carburettor faults, rather than carrying out the fundamental checks. (Wrongly adjusted tappets, incorrect valve timing, manifold air leaks, punctured diaphragms were some of the most common and most basic issues and the â€˜reasonâ€™ for wrongly changing carburettors â€“ ALWAYS to no effect!)
Whilst we had an Oscilloscope, which we used 8 times a day â€“ one hour allotted to each car â€“ our scope didn’t have the power and versatility of the PicoScope and the simple but immensely effective accessories which enable one to confirm mechanical faults so readily.
Impressive stuff: I very much enjoyed reading your report. I hope others will appreciate the importance of this article and start to take advantage of the potential of the Picoscope and its accessories in finding the origin of a multitude of problems without making a single assumption.
David Paterson

Jorge Vega R.January 20 2014

Thi engine is the same use in the Toyota GT 86 and the Scion FS. I found it very interesting even I don’t have the COP and the transducer.

Could you post the meaning of NSR and NSF in your Jargon?

Thanks.

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